The present invention relates in general to a fault-tolerant control system and pertains more particularly to a control system employing an input sensing network in combination with a signal processing system for eliminating false alarms and more particularly for eliminating the effect of radio frequency interference (RFI); electromagnetic interference (EMI); transients (voltage or current); and random false alarm signals.
Fire alarm systems are highly susceptible to false alarms due to the increased sensitivity of smoke detectors to RFI, EMI, transients and random false alarms due to air-velocity or other causes. Detectors are now calibrated to be more sensitive as they have to pass performance tests for a wide range of fires. With the greater use of digital electronics and power controls using various modulation techniques such as pulse width modulation and pulse position modulation, electromagnetic interference and radio frequency interference are steadily increasing problems. Load switching and the proximity of signalling wires to power conductors cause transients which are yet another problem for control systems.
A number of methods have been tried in an attempt to reduce the effect of electrically-induced signals. The usual methods have been to use suppressors, filters and integrators. However, because of the wide spectrum of these signals, the solutions have been both expensive and inadequate. Also, suppression, filtration and integration at the control do not eliminate false-alarms caused by sensors such as smoke detectors false-alarming due to air movement through an ionization chamber, vibration causing optical detectors to alarm, random, non-sustained passage of smoke, dust or condensation through a detection chamber. Electrical induction may also cause a detector to alarm, and suppressors, filters and other noise-reduction means at the control will not solve the problem. All unwanted signals--electrical or otherwise--will henceforth be referred to as "noise".
Another problem with control systems is that an adequate change in signal level has to occur for the system to differentiate between normal variations such as may be caused by signal-line impedance, voltage variations, system loading and an alarm signal caused by a sensor in alarm. In order to develop an adequate signal, the sensing network usually has to have an impedance which is high enough to develop the required voltage when an alarm current flows in the circuit. This impedance has a major drawback in that if more than one sensor is in alarm the voltage lost across this impedance reduces the voltage available to the signalling line. Because sensors usually have relays which control vital functions such as doors and elevators, dampers, vents, fans and other heating and ventilating devices, failure to supply adequate voltage can result in serious problems. For example, failure of a damper can cause the spread of smoke into other areas not directly effected by the fire. An even more serious condition may occur if some relays do operate and then drop out as the voltage drops further. When the relay load is shed, the voltage on the signal line rises and relays again begin to operate. This time, however, they may operate in a different sequence and cause smoke to be deliberately directed to other areas.
The method presently employed to get around this problem is to run separate wires to power the sensors or detectors and to run separate wires to detect the alarm signals. This leads to a great deal of additional expense because of the extra wiring, labor and circuits needed to supervise the power conductor. To economize, systems do not supervise the power conductor to each signaling loop separately and this makes it very difficult to locate the area of a fault.
Accordingly, it is an object of the present invention to provide a fault-tolerant system adapted to overcome all of the aforementioned problems described hereinabove.